1. Field of the Invention
This invention relates to a lens position control device for use in cameras, observation instruments, etc.
2. Description of the Related Art
Lenses adapted to be built in video cameras are generally of the type having four constituent lens groups as shown in FIG. 7.
In FIG. 7, 1 is a lens group (F) for focusing arranged at the front end of the lens barrel; 2 is a variator lens or lens group (V) for varying the image magnification; 3 is a compensator lens or lens group (C) for causing the sharply focused object to be properly positioned after the image varying operation; and 4 is a relay lens or lens group (R) for forming an object image. Incidentally, when in the position of FIG. 7, the zoom lens has its focal length set at the wide-angle end (shortest) and is focused on an infinitely distant object. For the purpose of explaining the movement of each lens group below, the places where the lens group 1 (F), the lens group 2 (V), and the lens group 3 (C) take when in this position are considered here to be at the zero point.
FIGS. 8(A) and 8(B) through FIG. 10 show the movement relationships of the aforesaid lens groups 1, 2 and 3 (F, V, C) with respect to the variation of the focal length of the entire system or the object distance. In the following, by reference to these figures, the features of the zoom lens will be described.
FIG. 8(A) is a graph with the lens group 2 (V) axially moved a distance shown in the abscissa to vary the focal length, f, of the zoom lens shown in the ordinate. FIG. 8(A) shows how the focal length f varies when the lens group 2 (V) moves, where W represents the wide-angle end at which the focal length of the zoom lens becomes shortest, and T represents the telephoto end at which the focal length of the zoom lens becomes longest.
FIG. 8(B) is a graph depicting the axial movement of the lens group 3 (C) along the abscissa and the focal length f of the zoom lens along the ordinate, FIG. 9(B) shows the variation of the distance the lens group 3 (C) has to move from the point 0 respect to the variation of the focal length.
FIG. 9 is a graph showing in the abscissa the reciprocal of the distance (in meters) from the camera to an object to be photographed, and showing in the ordinate the distance the lens group 1 (F) axially moves forward from the point 0. FIG. 9 shows the way in which the position of the lens group 1 varies with the variation of the object distance.
FIG. 10 is a graph showing in the ordinate the distance the lens group 1 (F) axially moves forward to effect focusing, and showing in the abscissa the focal length of the zoom lens FIG. 10 shows the relationship of the start point of the lens group 1 (F) with the focal length f at object distances of 1 m, 2 m, 3 m and infinity.
From all the above-identified figures, it is understandable that the publicly known zoom lens has the following features. That is, as is apparent from FIG. 9 and FIG. 10, in a case where the object distance does not change, the variation of the focal length by zooming does not require that the lens group 1 (F) be moved. Therefore, the lens group 2 (V) and the lens group 3 (C) may be interlocked according to the features shown in FIGS. 8(A) and 8(B). Thus, the position control of each lens group is relatively simple. Hence position control can be accomplished by cams or like mechanical control means.
FIG. 11 is a sectional view showing an interlocking mechanism for the lens group 2 (variator lens) and the lens group 3 (compensator lens) of the publicly known zoom lens. In FIG. 11, a lens group holder 5 containing the lens group 2 (V), and another lens group holder 6 containing the lens group 3 (C) are guided along a common optical axis by guide bars 7 and 8. A cam sleeve 9 has camming slots bored in the circumferential surface into which pins 5a and 6a (radially outwardly extending from the lens group holders 5 and 6) are inserted. A fixed tube 10 is fitted on the outer diameter of the cam sleeve 9 and is fixed to a stationary member such as the lens barrel. A zoom actuator ring 11 is fixedly secured to the cam sleeve 9 by a connection portion 11a and is fitted on the outer diameter of the fixed tube 10 so that only rotation relative thereto is possible. When zooming, as the zoom actuator ring 11 is rotated, the cam sleeve 9 also is rotated. As a result, the relative position of the pin 5a in the camming slot 9a and the relative position of the pin 6a in the camming slot 9b change to move the lens group holder 5 and the lens group holder 6 axially in differential relation.
However, as is evident from FIG. 9 and FIG. 10, in the conventional zoom lens, to focus on an object at a closest distance (for example, less than 1 m), the amount of forward movement of the lens group 1 (F) must be made great in proportion to the reciprocal of the object distance. Since focusing is effected just in front of the lens, and because the total focusing movement must be increased to almost infinity, it is impossible to take shots at very short distances. This constitutes a serious problem of the conventional zoom lens.
Therefore, in recent years, to make it possible to perform focusing without moving the first lens group 1 (F), a so-called inner focus type of zoom lens has been proposed. An example of this zoom lens is shown in FIG. 12. In this zoom lens, the lens group 1 and a front lens 4A of the relay lens group 4 are arranged not to move (as fixed lenses,) while the lens group 2 (variator) is arranged, likewise as in the former type of zoom lens of FIG. 7, to move when the focal length is changed. Also, a rear lens 4B (RR) of the relay lens group 4 (RR) has the compensating function like that of the compensator lens of the former type of zoom lens. By axially moving that lens 4B similarly to the conventional compensator lens, compensation is accomplished during zooming. And, the lens 4B is further given an additional function of adjusting focus. When performing only focusing, the lens 4B alone is moved.
Also, another example of an arrangement of a zoom lens of the inner focus type is shown in FIG. 15. In this case, four lens groups are used, of which the lens group 2 has the image magnifying function like to the conventional 4-group type of zoom lens of FIG. 7. However, what is different as compared with FIG. 7 is that the lens group 1 is fixedly secured to the fixed lens barrel 101. Because of this, the lens group 3 which would conventionally work only to compensate is obliged to also serve the focusing function.
In zoom lenses having such a lens arrangement, because of the lens group 1 is not movable, focusing can be effected down even to a very short object distance. Yet, since the relationship of the relative positions of the movable lenses, namely, the lens group 2 and the relay rear lens of FIG. 12, or the lens group 3 of FIG. 15 is extremely complicated, a the simple mechanism such as the cam mechanism of FIG. 11 cannot control the lens group 2 and the relay rear lens 4B of FIG. 12 or the lens group 3 of FIG. 15. Therefore, of a zoom lens of having lens arrangement shown in FIG. 12 or FIG. 15 using a mechanical operating means only is very difficult to achieve.
FIG. 13 is a graph depicting the position of the lens group 2 in the abscissa and the position of the relay rear lens 4B (RR) of the relay in the ordinate, representing the relationship of the relative positions of both lenses to each other at every object distance. As is apparent from FIG. 13, the relationship of the relative positions of both lenses changes as the distance of the object changes from infinity, 3 m, 1 m, 0.5 m, 0.2 m, 0.01 m. Therefore, it is understandable that it is impossible to control both lenses by a simple control mechanism such as the cams.
Nevertheless there has recently been made a proposal for employing a control method whereby the relay rear lens 4B alone is controlled relative to the lens group 2 in response to the detection result of whether or not an image is correctly focused on the focal plane in realizing the zoom lens of FIG. 12. And, even a commodity developed based on this proposal has been announced.
FIG. 14(A) schematically shows the lens position control method and lens construction and arrangement employed in that proposal and commodity, including a lens group 1, a lens group 2, a front lens 4A of a relay lens group, a rear lens 4B of the relay lens group, means 12 for sensing an image formed on the focal plane, a focus control (AF) circuit 13 for detecting when the image is in focus and, when it is out of focus, bringing it into focus, and drive means 14 whose operation is controlled by the AF circuit 13 to move the rear lens 4B of the relay lens group to the proper position.
FIG. 14(B) through FIG. 14(D) show an example of the automatic focus adjusting device. In FIGS. 14(B), 17 represents the entire area of a picture frame 17 of the video camera; and 18 represents a region from which a signal is taken out for the distance measuring purpose. Also, the object to be photographed is assumed to have a contrast 19. In FIG. 14(C), supposing that (a) is this contrast portion, then (b) shows a Y signal output, (c) the differentiated value of the Y signal, (d) its absolute value, and (e) a peak-held signal. Here, the height A represents the degree of focus. FIG. 14(D) shows the variation of the height A in the ordinate with the variation of the lens position of the lens group 1 of FIG. 7 or the lens 4B of FIG. 12, where the sharpest focus is established at the position B of the peak.
Incidentally, another improved method is proposed in Japanese Laid-Open Patent Applications Nos. Sho 62-296110, 62-284316 and others. This is to form position information of the variator lens and the compensator which also serves as the focusing lens, or position information of the variator lens and the distance operating member (distance ring), based on which the amount of movement of the variator lens is related to a number of units of movement of the compensator-cum-focusing lens (hereinafter called the "double-purpose" lens), whereby such a relationship is stored in a memory. Thus, each time a distance the variator lens is moved is given, the movement of the double-purpose lens is controlled in accordance with the corresponding number of units read from the memory (and out-of-focus information).
In the publicly known zoom lens and lens position control method shown in FIG. 14(A), if the accuracy and speed of the input signal of the AF circuit 13 from the image sensing means 12 are high, blurring and distortion do not take place in the image formed on the focal plane. But, because in actual practice the possibility of lowering the control accuracy of the rear lens 4B of the relay lens group by the response delay due to the performance of distance measurement in cycles and others, is very high, there is a serious drawback that a large blurring is apt to occur.
Further, in the above-described improved method, because the detection of the predetermined amount of movement of the variator lens becomes a prerequisite for a highly accurate movement of the aforesaid double-purpose lens to be obtained, there is a need to define the amount of movement of the variator lens much more finely than ever. Further, the moving speed of this double-purpose lens must be made faster. Otherwise it would take a long time to correct the produced blurring.